Fix use of deleted assignment operator for gDirectory under ROOT 6.24 in StFwdTrackMaker (#757)

Replaced `gDirectory = prevDir` with `prevDir->cd()` to avoid
compilation error with ROOT 6.24

```
.sl73_x8664_gcc485/OBJ/StRoot/StFwdTrackMaker/StFwdQAMaker.cxx: In member function 'virtual int StFwdQAMaker::Finish()':
.sl73_x8664_gcc485/OBJ/StRoot/StFwdTrackMaker/StFwdQAMaker.cxx:196:20: error: use of deleted function 'ROOT::Internal::TDirectoryAtomicAdapter& ROOT::Internal::TDirectoryAtomicAdapter::operator=(const ROOT::Internal::TDirectoryAtomicAdapter&)'
         gDirectory = prevDir;
                    ^
In file included from /cvmfs/star.sdcc.bnl.gov/star-spack/spack/opt/spack/linux-rhel7-x86_64/gcc-4.8.5/root-6.24.06-ojgeqx4c6cazamrg2mltgfm3wpivwzyf/include/TDirectoryFile.h:25:0,
                 from /cvmfs/star.sdcc.bnl.gov/star-spack/spack/opt/spack/linux-rhel7-x86_64/gcc-4.8.5/root-6.24.06-ojgeqx4c6cazamrg2mltgfm3wpivwzyf/include/TFile.h:28,
                 from StRoot/StChain/StMaker.h:20,
                 from StRoot/StFwdTrackMaker/StFwdQAMaker.h:11,
                 from .sl73_x8664_gcc485/OBJ/StRoot/StFwdTrackMaker/StFwdQAMaker.cxx:1:
/cvmfs/star.sdcc.bnl.gov/star-spack/spack/opt/spack/linux-rhel7-x86_64/gcc-4.8.5/root-6.24.06-ojgeqx4c6cazamrg2mltgfm3wpivwzyf/include/TDirectory.h:247:11: note: 'ROOT::Internal::TDirectoryAtomicAdapter& ROOT::Internal::TDirectoryAtomicAdapter::operator=(const ROOT::Internal::TDirectoryAtomicAdapter&)' is implicitly deleted because the default definition would be ill-formed:
    struct TDirectoryAtomicAdapter {
           ^
/cvmfs/star.sdcc.bnl.gov/star-spack/spack/opt/spack/linux-rhel7-x86_64/gcc-4.8.5/root-6.24.06-ojgeqx4c6cazamrg2mltgfm3wpivwzyf/include/TDirectory.h:247:11: error: non-static reference member 'std::atomic<TDirectory*>& ROOT::Internal::TDirectoryAtomicAdapter::fValue', can't use default assignment operator
cons: *** [.sl73_x8664_gcc485/OBJ/StRoot/StFwdTrackMaker/StFwdQAMaker.o] Error 1
cons: errors constructing .sl73_x8664_gcc485/OBJ/StRoot/StFwdTrackMaker/StFwdQAMaker.o
```

Author: plexoos

StRoot/StFwdTrackMaker/StFwdQAMaker.cxx

@@ -182,18 +182,18 @@ int StFwdQAMaker::Finish() {
 
     //beginning new
     if ( mLocalOutputFile != "" ){
-        auto prevDir = gDirectory;
+        TDirectory* prevDir = TDirectory::CurrentDirectory();
 
         // output file name
         TFile *fOutput = new TFile(mLocalOutputFile, "RECREATE");
         fOutput->cd();
         for (auto nh : mHists) {
-            nh.second->SetDirectory(gDirectory);
+            nh.second->SetDirectory(TDirectory::CurrentDirectory());
             nh.second->Write();
         }
 
         // restore previous directory
-        gDirectory = prevDir;
+        prevDir->cd();
 
         LOG_INFO << "Done writing StFwdQAMaker output to local file : " << mLocalOutputFile << endm;
     }//end new

StRoot/StFwdUtils/StFwdAnalysisMaker.cxx

@@ -28,18 +28,18 @@ StFwdAnalysisMaker::StFwdAnalysisMaker() : StMaker("fwdAna"){
 int StFwdAnalysisMaker::Finish() { 
 
     if ( mLocalOutputFile != "" ){
-        auto prevDir = gDirectory;
+        TDirectory* prevDir = TDirectory::CurrentDirectory();
 
         // output file name
         TFile *fOutput = new TFile(mLocalOutputFile, "RECREATE");
         fOutput->cd();
         for (auto nh : mHists) {
-            nh.second->SetDirectory(gDirectory);
+            nh.second->SetDirectory(TDirectory::CurrentDirectory());
             nh.second->Write();
         }
 
         // restore previous directory
-        gDirectory = prevDir;
+        prevDir->cd();
 
         LOG_INFO << "Done writing StFwdAnalysisMaker output to local file : " << mLocalOutputFile << endm;
     }